Electronics > Metrology

precise +-1500 volt gigaohm input voltmeter

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alireza7:
hi i am making on a precise high voltage gigaohm input voltmeter.
i found out this topic :
https://www.eevblog.com/forum/metrology/how-a-precision-dmm-have-a-high-input-impedance-in-the-gohm-rang/
and this schematic:
https://www.eevblog.com/forum/metrology/how-a-precision-dmm-have-a-high-input-impedance-in-the-gohm-rang/?action=dlattach;attach=260049;image

it is my schematic but i didn't assemble varistors and gas tube:



there is two problem in my circuit:

1-i figure out with those series 1k resistor in the input the buffer do not follow the input
2- when i bypassed these resistor the output seems to follow the input but it's not precise and it have drift and also it's drift is not linear

what should i do to get the exact input at the output of buffer?

also an other question: if i assemble varistors and gas tube ? do they draw leakage current and influence the input and  decrease the input impedance?is there a better way for protection?

e61_phil:
Do you want to measure 1kV with an input impedance of ~1G?

Why do you build such a complicated buffer and not simply a voltage divider?

alireza7:

--- Quote from: e61_phil on October 18, 2017, 01:27:31 pm ---Do you want to measure 1kV with an input impedance of ~1G?

Why do you build such a complicated buffer and not simply a voltage divider?

--- End quote ---

i want to measure +-1500v with an input impedance of 1gigaohm or higher

your answer is in this link please read this topic:
https://www.eevblog.com/forum/metrology/how-a-precision-dmm-have-a-high-input-impedance-in-the-gohm-rang/

Alex Nikitin:

--- Quote from: alireza7 on October 18, 2017, 03:32:56 pm ---
--- Quote from: e61_phil on October 18, 2017, 01:27:31 pm ---Do you want to measure 1kV with an input impedance of ~1G?

Why do you build such a complicated buffer and not simply a voltage divider?

--- End quote ---

i want to measure +-1500v with an input impedance of 1gigaohm or higher

your answer is in this link please read this topic:
https://www.eevblog.com/forum/metrology/how-a-precision-dmm-have-a-high-input-impedance-in-the-gohm-rang/


--- End quote ---

Just listen to e61_phil - use a 9.99Gohm HV resistor in series with a 10Mohm meter at 2V range. That's it - you have a high voltage 2kV scale meter with 10Gohm input resistance. Simple, safe and much cheaper  ;) .

Cheers

Alex

HighVoltage:

--- Quote from: Alex Nikitin on October 18, 2017, 03:47:20 pm ---
Just listen to e61_phil - use a 9.99Gohm HV resistor in series with a 10Mohm meter at 2V range. That's it - you have a high voltage 2kV scale meter with 10Gohm input resistance. Simple, safe and much cheaper  ;) .

Cheers

Alex

--- End quote ---
That is really good advise.

Most GOhm resistor are only good for a few hundred volts and when they break from over voltage, they can shorten out. This could potentially damage the volt meter.
 
I would use 10x 1GOhm in series and then the 10 MOhm resistance of the meter.
And with 10 pieces of GOhm resistors, you can match them until you have 9.99 GOhms

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